This invention pertains to the art of large centrifugal fans which operate in elevated temperature ranges.
Some hot gas handling fans are used in arrangements in which two or more of the fans are arranged in a parallel flow arrangement. One example of the use of fans in such a parallel arrangement would be in a gas recirculation system in connection with an electric utility boiler and furnace where a portion of the flue gas from the furnace is recirculated back to the lower part of the furnace for the purpose of maintaining mass flow through the boiler to better control steam temperature. Under some load conditions of the power plant, only one of the two fans may be operating. However, with a changed fan load requirement, such as an increase, the additional fan will be required to operate. Since bringing the additional fan on line with little or no advance warning is often necessary, it is helpful if it is held at a temperature sufficiently close to the temperature of the hot gas which it is to handle, so as to avoid unacceptable vibration which can occur if the wheel of the fan is thermally distorted by heating the wheel too fast by the high-temperature hot gas. Other examples in which multiple fans in parallel handle high-temperature dirty gas, and where the fans are selectively operated and it is desirable to maintain the non-operating fans at a temperature higher than ambient, are found in certain industrial processing such as coal shale processing and pelletizing.
It is the aim of this invention to provide a large centrifugal fan scroll construction, and system, in which any non-operating hot gas fan of a number of hot gas fans is maintained at an elevated temperature to permit rapid start-up of operation.